I just got my 27" iMac a few days ago (they actually had the EXACT build I wanted at the Apple store, so I didn't have to order it!). I too am a virgin to Mac; I have been a power PC user and gamer since the late 90's, and never thought I'd ever buy a Mac.
It all started after I owned 2 Android phones in a row. I am fickle with my tech toys, so I eventually got bored with the Android OS and wanted to see what all the iPhone hype was about since Verizon had finally got them on their network...that was about a year ago. I then bought the Apple TV, then an iPad, because I loved the way they all natively worked together.
My PC hardware had been getting more and more obsolete the past couple of years, and I wasn't able to run Battlefield 3 on it, so I had been thinking about upgrading the hardware. Since I already owned so many Apple products now, I started considering an iMac because I love the way it looks and the fact that it is an all-in-one unit. After 3 days of ownership, I am extremely happy with it and have got over my buyers remorse for spending the $2,500 that I dropped on the high-end model!
My advice is to do what I did and immediately use Bootcamp and install Windows on a hard drive partition. This is like my security blanket, because I know I still have the OS that I have used for so long just a few clicks away. After the Bootcamp install, I also installed VMWare so I can access the Windows partition through OSX as well. However, you'll want to actually restart and boot into Windows for PC gaming. By the way, Battlefield 3 runs GREAT on the Windows partition!
It's taking some time for me to learn how to navigate through the OSX files, but I'm quickly learning because OSX organizes everything so well...I think everything is beginning to click and its starting to come a little more naturally. If things keep going this well, I don't see myself building anymore PCs in the future!
I have also been checking out forums like this one and online articles to try to pick up tips and tricks, and just to learn more about how OSX works. Surprisingly, the basic principles are still there...it's kind of like the speaking the same language, but with a thick accent.
Good luck with your new computer!